88 IV. N. LOGAN 



so in places that they might with propriety be called clay, and 

 are variable in color. The prevailing color is dark blue. Here 

 and there in the shales are thin beds of limestone containing 

 Inoceramns labiatus and species of cephalopods. Ostrea congesta, 

 var. Bentonensis attached to Inoceramus sp., is the most abundant 

 species. Fish teeth, sharks' teeth, and pavement plates and 

 fossil wood containing species of Parapholce, also occur. Near 

 the upper limit the shales contain a species of Inoceramus with 

 Serpula pla7ia attached. The thickness of the Ostrea shales in 

 the Kansan area is 1 50 feet. They are the stratigraphical equiva- 

 lent of the lower Carlile shales in the eastern Colorado area. 

 The Black Hill area possesses a bed of shales twenty or thirty 

 feet in thickness, which is stratigraphically and palaeontologic- 

 ally equivalent to the Ostrea shales. Specimens of Serpula plana. 

 and Inoceramus labiatus were found in an outcrop of the shales 

 on Hat Creek, a southern branch of the Cheyenne River. The 

 Ostrea shales in this area are somewhat arenaceous, and the 

 prevailing colors are blue and light yellow. The Ostrea 

 shales are wanting in the Iowa-Nebraska and eastern Dakota 

 areas. 



The Blue Hill shales. — The Blue Hill shales form the upper 

 zone of the shale group. They are dark or slaty colored shales 

 which, under the influence of weathering, break up into very fine, 

 chaff-like fragments which are so light as to be moved about 

 somewhat easily by the wind. On account of their fine, lami- 

 nated appearance the shales are sometimes called paper shales. 

 They are unfossiliferous and homogenous, except in the upper 

 third. This zone has numerous argillaceo-calcareous concretions 

 called septaria imbedded in its shales. Some of these concre- 

 tions possess the cone-in-cone structure, while others are formed 

 of concentric layers. Fissures in others of the concretions have 

 been filled in by sedimentation with calcite which ranges in color 

 from white to claret. These are the true septaria. Many of the 

 concretions are highly fossiliferous. The following species have 

 been collected from them : Scaphites larvceformis, S. vermiformis, 

 S. warreni, S. ventricosus, S. mulla?ianus, Rostellites willisto?iii, Pri- 



